1. Field of the Invention
The invention relates generally to the field of overvoltage protection devices for electronic circuitry and more particularly to a device and method of fabricating overvoltage protection devices using PiN diodes.
2. Discussion of Related Art
Overvoltage protection devices are often used in conjunction with integrated circuits because excessive overvoltage can destroy or impair elements of an integrated circuit. For example, an overvoltage event may cause a metal melt, junction breakdown or oxide failure. Overvoltage events can also lead to latent defects in an integrated circuit that may shorten the lifespan or substantially lessen the performance of an integrated circuit. The goal of overvoltage protection devices is to prevent damage to the integrated circuit when an overvoltage event occurs. Electrostatic discharge (ESD) is a major source of overvoltage events. ESD is usually caused by one of three events: direct electrostatic discharge to the device, electrostatic discharge from the device, or field-induced discharges. The likelihood of damage due to ESD is a function of the device""s ability to dissipate the energy of the discharge or withstand the voltage levels involved. This is known as the device""s xe2x80x9cESD sensitivity.xe2x80x9d
Conventional integrated circuit overvoltage protection devices utilize Zener diodes, PN diodes, gate grounded MOSFETS or SCR devices to substantially isolate integrated circuits from overvoltage events. These devices have been successfully employed in traditional integrated circuits to limit damage due to overvoltage.
Radio frequency (RF) integrated circuits (operating in the approximate range of 900 MHz to 5.8 GHz) and other high frequency integrated circuits present a particular challenge. Conventional integrated circuit overvoltage protection devices are unsuitable because their large input capacitance acts roughly as a high-pass filter. High frequency signals are sensitive to the capacitance of overvoltage protection devices. A higher frequency signal requires a lower input capacitance from the overvoltage protection device.
SCR devices were thought to be promising for RF applications because of their high ESD performance per micron. However, the capacitance of SCR devices, measured at 150 fF, however, is still too high for acceptable use in RF applications. Zener diodes, PN diodes and other conventional forms of ESD protection have all failed to produce reverse bias capacitance that are sufficient for use in RF integrated circuits.
In accordance with one embodiment, an overvoltage protection device operates by shunting an overvoltage event through a number of PiN diodes away from a signal node of an integrated circuit. Within a specified voltage range, the PiN diodes are in a reversed biased mode. If the voltage moves outside the specified voltage range, some or all of the PiN diodes switch from reverse bias mode to a forward biased mode to shunt the undesired and potentially harmful voltage away from an internal circuitry node. The overvoltage protection device is useful to protect radio frequency (RF) integrated circuits and other high frequency integrated circuits because the input capacitance of the PiN diodes in reverse bias mode is approximately 0.2 femtoFarads (fF) per micron of diode periphery. This low capacitance allows high frequency signals to pass through the overvoltage protection device without significant signal attenuation or signal degradation.
In one illustrative embodiment, one or more positive voltage PiN diodes are arranged such that in forward bias a highly positive voltage event will be shunted to a positive voltage source. One or more negative voltage PiN diodes are also arranged such that in forward bias mode a highly negative voltage event will be shunted to a negative voltage source. A Zener diode may be placed between the positive voltage source and negative voltage source whereby a highly positive voltage event would xe2x80x9cbreakdownxe2x80x9d the Zener diode. Upon xe2x80x9cbreakdownxe2x80x9d of the Zener diode, the highly positive voltage event would be further shunted to the negative voltage source. By serving as a ground, the negative voltage source would act to dissipate the highly positive voltage event.
In another illustrative embodiment, the overvoltage protection device is fabricated on a silicon-on-insulator (SOI) wafer. The process of manufacturing PiN diodes on a SOI wafer is a subset of the process of manufacturing CMOS transistors on a SOI wafer. Thus, the overvoltage protection device can be implemented on the same SOI wafer as the RF integrated circuit without additional manufacturing costs.